Add Asynchronous UART implementation, which does not drop data
when automatic hardware-flow-control is set in the device tree.
With automatic hardware flow control, the CTS pin will be
automatically deactivated when there are no more asynchronous
UART RX buffers available. After buffer space becomes available,
and UART RX is restarted, the CTS pin will be activated.
Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>
10bd2de235